In recent times, due to the growing concern toward the environment, the steep rise in crude oil prices, and the like, with regard to vehicles, the development of various techniques for improving fuel consumption has been advanced. One example thereof includes the utilization of regenerative energy in which kinetic energy is converted into electric energy and the energy is accumulated in a battery to be utilized. And then, with regard to the battery used in an idling stop vehicle (an IS vehicle), since power for the electric load at the time of the IS must be supplied from the battery and the number of times of restarting an engine is large, the discharge amount thereof is increased as compared with that in the case of being used in a conventional engine vehicle. As such, accepting a larger amount of regenerative energy in a short period of time is one of important characteristics of the battery. In general, the lower charged state, namely, the more partially charged state (PSOC (Partial State of Charge)) the battery is in, the higher the regenerative charge accepting performance becomes. As such, a battery requiring the performance for accepting regenerative energy is frequently used in a PSOC-controlled state.
Since the discharge amount of the battery for an IS vehicle is larger than that of the battery for a conventional engine vehicle as mentioned above, and moreover, the battery for an IS vehicle is used in a PSOC-controlled state, the battery for an IS vehicle must be more excellent in durability than the conventional battery. As a means of enhancing the durability, it is common to increase the amount of an active material in the positive or negative electrode. However, since the ratio of the amount of the electrolyte solution to the amount of the active material is reduced when the amount of the active material is increased, the concentration of the electrolyte solution is easily lowered at the time of discharging. When the concentration of the electrolyte solution is lowered, the solubilities of lead and lead sulfate are increased, and lead is allowed to ionize and to be easily eluted from the electrode plate. The eluted lead ions grow into needle-like crystals at the time of the subsequent charging, and a permeation short circuit in which the crystals penetrate through a separator and result in a short circuit is easily caused.
Incidentally, in the lead-acid battery, when the charge-discharge is repeated, water is produced at the time of discharging and thick sulfuric acid is produced at the time of charging. And then, since the thick sulfuric acid has a higher density than water, is allowed to move downward and is easily settled, a phenomenon called stratification in which the upper and lower portions of the electrolyte solution are different from each other in (sulfuric acid) concentration occurs. In a conventional engine vehicle, since the battery is overcharged at the time of driving, on this occasion, the stratification is alleviated by the stirring action in the electrolyte solution caused by oxygen and hydrogen gases generated from the positive and negative electrode plates. However, under a PSOC-controlled state, since the battery is charged at the time of deceleration, the charging time is extremely short, and the poor charged state continues, the stirring action in the electrolyte solution caused by oxygen and hydrogen gases is not developed and the stratification easily occurs (Patent Document 1). When the stratification occurs, since the electrolyte solution concentration in the upper portion of the cell is lowered, a permeation short circuit at the upper portion of the cell is easily caused.
As stated above, in the flooded lead-acid battery for an IS vehicle, for architectonic reasons (the amount of the active material is relatively larger than the amount of the electrolyte solution) and operational reasons (easily stratified), a permeation short circuit is easily caused as compared with the battery for a conventional engine vehicle.